Many automotive hybrid electric vehicles utilize the “parallel hybrid configuration” where an electric motor/generator is used in the vehicle driveline. The typical motor/generator is a permanent magnet brushless type that is often integrated into the transmission housing. During vehicle braking, the unit operates or functions as a generator, providing braking torque and conserving vehicle kinetic energy as “regen” or stored generated energy in batteries or other means. When the driver commands acceleration after a stop, this stored electrical energy runs the motor to provide additional torque to the vehicle drive system, saving fuel and allowing a smaller engine to be utilized.
When the vehicle is traveling at a constant speed, the motor/generator permanent magnet rotor is rotating but is normally de-energized and idle and the vehicle's internal combustion engine is the primary source of power. This highlights the principal difference in motor/generator function between the parallel hybrid and a series hybrid or pure electric vehicle. The typical parallel hybrid system operates the motor/generator intermittently and has significant periods of inactivity during constant speed operation where magnet induced iron losses induce parasitic drag torque to the vehicle. In series hybrid or pure electric drive systems, the motor/generator is the vehicle's prime accelerator and the motor/generator duty cycle parallels the vehicle speed. When the motor/generator is idle in the series hybrid or pure electric vehicle case, the vehicle is at rest. Because of this, significant periods of magnet induced iron losses are eliminated when the motor/generator is the vehicle's prime accelerator.
The present invention is directed to a method of eliminating magnet induced torque losses of permanent magnet motors/generators that are used in parallel hybrid configurations.
During the constant speed cycle, even though the stator coils of the motor/generator are de-energized, the spinning high strength magnets in the motor create a parasitic torque drag on the vehicle. Whenever rotating magnetic fields cut through the electrical steel and iron materials from which the stators are normally made, the magnetic flux densities produce “iron losses” that dissipate energy as heat. The iron losses generally vary as square functions of the frequency and the magnetic flux density or strength. At high rotational speeds, the iron losses of a permanent magnet machine can require considerable power levels that cause parasitic drag and stator heating. This parasitic drag detracts from the overall efficiency and fuel savings of the parallel hybrid system. A driving cycle involving prolonged constant speed highway and long distance driving will sacrifice fuel economy due to the significant power consumed by the iron losses of the motor/generator.
Many embodiments of this parallel hybrid vehicle technology rotate the motor/generator whenever the vehicle is moving. In many cases this is merely done for simplicity and cost savings. Methods to modify and reduce the parasitic drag of iron losses must change either the rotational speed of the magnets or reduce the magnetic strength of the permanent magnet rotor. If reductions of iron losses can be achieved during long term, constant speed operation, then parasitic torque drag on the vehicle will reduce and fuel economy will be improved.
A clutch arrangement can be used to de-couple the motor/generator from the vehicle drive line during “de-energized” periods. This is not common, due to the cost and durability issues involved. In the field of electric vehicle traction motors, several patents teach methods of permanent magnet field weakening to achieve an extended speed range. U.S. Pat. Nos. 6,492,753 and 6,555,941 to Zepp et al. disclose a magnetic field weakening method that axially offsets the internal permanent magnet rotor from the electrical steel laminations of the stator for extended speed range. Also U.S. Pat. No. 6,943,478 to Zepp et al. discloses a magnetic field weakening method that axially offsets the external permanent magnet rotor of a hub motor from the electrical steel laminations of the internal stator for extended speed range.
U.S. Pat. No. 6,844,647 to Horber discloses a permanent magnet rotor that includes an inner sleeve and an outer sleeve with varying positional relationships that vary magnetic flux.
U.S. Pat. No. 6,137,203 to Jermakian et al. discloses a method of magnetic air gap adjustment to achieve extended speed in an axial gap type motor.
The primary focus these patents is the variation of permanent magnet flux to extend motor speed and constant power operation. When these prior art techniques are applied to parallel hybrid systems, their field weakening will reduce, but not eliminate, the parasitic torque drag characteristic commonly found during constant speed operation.
The present invention is directed to a method and system for eliminating parasitic torque losses that are caused when rotating magnets pass electrical steel when a motor/generator unit is not energized during constant speed operation of a vehicle.